Radio receiver with pulse sharpening system



E K. STODOLA RADIO RECEIVER WITH PULSE SHARPENING SYSTEM Oct. 10, 1950 2 Sheet-Sheet 1 Filed June 24, 1943 FREQ PROTEC- TOR DISC-R.

TO AME24 RECI SWEEP GEN.

FILTERS B RECT. 37

"*VOLTAGE GEN MOD. Mn-rER PULSE FROM TUNER 22 INVENTOR EDWIN K. STODOLAj BY I I H- RECT.

FIGA.

ATTORAEY Oct. 10,1950 E. K. ,STODOLA RADIO RECEIVER wrm PULSE SHARPENING SYSTEM Filed June 24, 1945 2 Sheets-Sheet 2 AMP PULSE INVENTOR EDWIN K. STODQLA ATIURNE)" PULSE AMP BY WM QM FIGS.

FIG]- m R R m o m TW -hm M m X R.

Patented Oct. Id, 195

UNITED STATES OFFICE RADIO RECEIVER WITH PULSE SHARPENING SYSTEM Edwin K. Stodola, Neptune, N. 3., assignor to the United States of America as represented by the Secretary of War (Grantedunder the act of March 3, 1883, as amended April 30, 1928; 3'70 0. G. 757) Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates .to frequency discriminating methods and circuits, and their application to modulated carrier wave systems. Systems to which this invention has been found particularly applicable are those used in connection with the pulse-echo method of object location or altitude determination.

In conventional systems of this type, short trains of wave energy, usually radio waves, are transmitted'at recurring intervals; On striking a target the waves are reflected or reradiated toward their source and the observed time interval between the transmitted waves and the echoes is a measure of the distance to said target.

In order to obtain accurate definition of the sharp pulses generated in such systems it is necessary to use receivers havingrelatively'wide band channels. The response of suchchannels is often appreciable for frequencies far removed from the channel actually required for transmission of the desired pulses. This feature causes considerable interference when a plurality of units operating on nearby frequencies,- which are within the acceptance band of said channels, are located in the same area. Since such units are not pulsed synchronously, this gives rise to a-plurality of pulse images continuously moving on the oscilloscope screen of any one unit, which obscure the desired signals, thus causing considerable difficulty in keeping track of said signals and resulting in quick fatigue of the operating personnel. It also makes possible deliberate'jamming by the enemy through the'use of frequency modulated or amplitude modulated signals which are'within the acceptance band of the receiver channels.

It is an object of this invention to provide an improved frequency discriminating circuit which will produce an output of one type in response to currents of one frequency or in a desired frequency region and produce an output of an'opposite type for frequencies on either side of said region.

A further object of the invention is to provide a circuit for developing an output of one polarity for signals in a restricted frequency band, and an output of the opposite polarity for'signals on either side of said band.

Another object is the application of the circuits mentioned in the foregoing objects to a wide band pulse receiver ystern whereby the response of said system is conditioned by the frequency of the r;

2 incoming signal. More specifically, signals of a frequency in or near the center or mean frequency region of the receiver channel will be more strongly or distinctly indicated than signals-on either side of said region.

Still another object of the invention is to provide a frequency responsive network for use with a wide band receiver system, said network developing a voltage of one polarity in response to signals in or near the center frequency region of said receiver and a voltage of an oppositepolarity for signals on either side of said region, and controlling the response of said system as a function of the polarity of said voltage.

In accordance with one system incorporating the invention the dual polarity output of the abovementioned frequency responsive network is applied to the deflecting electrodes of an oscilloscope, whereby signals of the desired frequency deflect the base line in one direction while undesired frequencies deflect said base line in the opposite direction.

In accordance with another system incorporating this invention all signals passing through the receiver are applied to one type of beam control electrode of the'oscilloscope, usually a deflecting electrode. The incoming signals are also applied to the abovementioned frequency responsive network, the output of which is applied to another type of beam control electrode of the oscilloscope, usually the beam intensity control grid. If the output is of the polarity produced by the signals in the center frequency region of the receiver channels, said grid will be rendered more positive so that the beam brightness will be increased. Signals outside this region will produce signals of opposite-polarity so that the beam will be cutoff or reduced in brightness.

For a better understanding of'the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, wherein like parts are indicated by like reference numerals, and its scope will be pointed out in the accompanying claims.

In the accompanying drawings:

Figure 1 is a block diagram of a circuit incorporating the invention;

Figure 2 is a block diagram of one form of the invention;

Figure 3 is a graph illustrating the operation of the circuit in Figure 2;

Figures 4 and 5 are circuits, partially schematic and partially in block diagram form; of other systems incorporating the invention;

otherwise conventional type of pulse-echo system,

including a transmitter channel A and receiver channel B, both coupled to a common directional antenna C, although separate antennas may be used. The output of the receiver feeds into an indicating oscilloscope D, the beam of which is periodically displaced by means of a sweep voltage from channel E. A synchronizing oscillator S justin-g it so that the transmitted pulse indication 26 is positioned at a given datum position 28 of the trace. The pointer is then disconnected from the phase. shifter, reset to zero position on scale 29, and' then'rjeconnecte d to the phase shifter so that zero scale reading represents the datum position. The phase shifter is then readjusted until the received pulse indication 2! is moved to thesame datum position. The transit time of synchronizes the transmitter and sweep voltage a channels. I

Synchronizing oscillator S delivers a sinewave, which is generally in the audio frequency region, to a pulse generator Iii, which, at each cycle of voltage from oscillator S, generates a sharp voltage pulse of considerably shorter duration than said cycle. ator I0 is a series of sharp pulses of very short duration spaced at intervals of considerably longerduration. Pulse generator I0 may be of the type which merely distorts the, sine Wave into a pulse of the desired shape or of the multivibrator type which is periodically keyed by the sine wave voltage, both types being well known in the art.

The pulses from generator l0 periodically key amodulator III'which is normally biased at or near cutoff. The modulator in turn keys a normally blocked radio frequency transmitting oscillator I2, which generates trains of oscillations.

for the duration of each pulse.

The output of oscillator S is also applied to sweep voltage channel E comprising an adjust-- able phase shifter I3, pulse generator I4, and sweep generator I5. Pulse generator I4 is generally similar to pulse generator I0 and delivers scope indicator D. Said oscilloscope includes a cathode H, a beam intensity control grid I8, vertically deflecting plates I9, and a fluorescent indicating screen 20. Conventional focusing electrodes and potentials therefor (not shown) are also part of said oscilloscope. By adjusting phase shifter I3, any desired point of the oscilloscope sweep can be synchronized with the pulsing of transmitter I2.

7 The pulses of R. F. energy from transmitter I2 areradiated through antenna C and also applied to receiver B. The energy radiated from the antenna, will upon striking an object, be reflected or reradiated back toward the antenna and impressed upon the receiver. Both the trans mitted and reflected pulses appear in the receiver output and are applied to the verticall deflecting plates IQ of oscilloscope D so that they vertically deflect the oscilloscope trace. Due to the time in transit of the received pulses, the deflections due to the main transmitted pulse 26 and the reflected pulse 21 will appear separated at a distance proportional to said transit time and hence the distance to the'refiecting object.

The distance of said target can be indicatedby means of suitable calibrations on the screen of the oscilloscope. Or, the distance can be measured by calibrating phase shifter I3 and ad- The resultant output of pulse gener-- type whereby desired signal frequencies yield an- .a minimum of loss.

the reflected signal can then be determined by noting the new position of the pointer on the scale. Since said transit time is the equivalent of aphase shift, scale 29 can be calibrated directly in terms of distance. For further details of this -method, reference is made to the applic'a tion .of S. H. Anderson, Serial No. 470,376, filed December 28, 1942.-

The receiver channel B includes a protector 2| between the antenna and tuner 22. The protector generally consists of a spark gap network which breaks down during the pulse transmission, because of the relatively large amplitude of the transmitted pulse, and greatly reduces the signal input to the receiver. When the transmission ceases, the gap recovers. andlpermits the relatively weak reflected signal to passto the receiver with Such networks. are .well known and do not invention Tuner 22, tuned to the received signal is g enerally of the superheterodne type but it may be.

of straight tuned R. F. type, Theoutput of the tuner may then be detected and the resulting pulse amplified and appliedto the oscilloscope.

As thus far described, the system is conventional. For reasons above set forth the receiver channels must be broadly tuned to avoid pulse distortion and hence interfering signal frequen-.

cies closely adjacent to the desired frequencyJget through the receiver.

In accordance with one form of the present invention, the output of tuner 22 is passed through a detector 23 of the frequency discriminating output of one polarity, and frequencies outside the desired range yieldan output of an opposite: polarity. This output is amplified bypulse amplifier 24' and appliedto the deflecting plates-of an oscilloscope, so thatdesired signal frequen cies appear above the baseline of the oscillo scope, while undesired frequencies appear below the base line, as indicated by 21- on the oscilloscope'screen. v

For a more detailed quency responsive detector ordiscriminator 23, reference is made to Fig. 2. The output of tuner 22 is passed through two. channels G "an d' I-I.

Channel G includes an adjustable attenuator- 3!) a filter 3|, and rectifier 32. Channel H includesan adjustable attenuator 33, a filter 34, and rectifier 35. alike with the exception of the filters; Filters 3| and 34 are both tuned to the mean or center frequency of the tuner 22, said frequency being the desired intermediate frequency in the case of a superheterodyne receiver. ever, a narrower pass-band than filter 34, as indicated by the resonance curves thereon. The

pass-band of filter 34 is'aboutthe same as that of the receiver tuner 22 so that it will pass the entire spectrum in the receiver output while the pass-band of filter 3| is as narrow-as de'sired, depending upon the frequency range atwhich re-x sponse is desired. The rectified outputs across load resistors 36 and 3'! are combined in phase per se form any partof this:

description" of the fre- The components of both channels 'are' Filter 3| has, how-- opposition. The time constant of rectifiers 32 and 35 must be suificiently small to follow the pulseenvelope.

The resultant output between lead 38 and ground is therefore the difference between the outputsof both channels and is represented by the curves shown. in Fig. 3, wherein the abscissa indicates'vari'ation in frequency of the input potentials and the ordinate indicates polarity and magnitude of the resultant voltage. Line F representsthe center frequency of tuner 22 and filters 3i and 34. Lines FI and F2 represent the band-pass near the peak of the resonance curve of filter 3| while F3 and F4represent the bandpass 'of tuner 22 and filter 34.

By adjusting attenuators and 33 so that the outputs across resistors 36 and 31 are equal at the center frequency F, the resultant output will be as shown by the solid curve 40. This curve shows that at thecenter frequency F and within a restricted frequency range Fl and F2 on either side of the center frequency the resultant output is substantially zero or minimum. By adjusting the attenuators so that the potential across resistor 36, at the center frequency F, is greater than that across resistor 31, the resultant potential, as shown by 'curve 4! is positive between the narrow limits F! and F2, where the desired signal frequency is located, and negative up to limits F3 and-F4, where interferingsignal frequencies are located. Such resultant potentials in the output of detector network 23 are amplified at 24, and applied to the vertically deflecting electrodes of the oscilloscope. Thus signals in the region between F1 and F2 will appear above the base line of the oscilloscope while the undesiredsignal frequencies would appear below the base line. This helps to eliminate the confusion caused by the continuously moving interfering signals.

Although the resultant output of the frequency discriminator circuit in Fig. 2 is positive for the center frequency region and negative for signals outside the region, the reverse polarity relation can be obtained by reversing the polarity of the rectifier elements in networks 32 and 35. Furthermore, no matter what polarities of the resultant voltages areapplied to the inputof pulse amplifier 24, the polarities in the output of the pulse amplifier will depend upon the number of amplifier stages. Thus if the resultant voltage output of detector 23 is positive for desired signal frequencies, the polarity of the output of amplifier 24 will be negative if an odd number of stages is used.

The interference reducing qualities of the system shown in'Figs. l and 2 depend upon the sharpness of response of the narrow band filter 31. To get a maximum of interference reduction, filter 3| should be as selective as possible, consistent with requirements of the side band components of the pulse modulation.

Another method of using the frequency discriminator is illustrated in Fig. 4. This circuit shows the output portion of the circuit in Fig. 2, rectifiers 32, and their load resistors 36, 3'! representing the corresponding parts of Fig. 2.

The oscilloscope intensity grid I8 is normally biased by a negative potential from source Ell through a high resistance 63, this bias being, however, insufficient to completely suppress the beam. The voltage across resistor 31, which is derived from all signals which pass through broadband filter 34, is amplified at SI and applied to the deflecting plates of the oscilloscope. Since filter 34 has a'band pass as wide as the tuner channel, all signals will pass through with -a minimumof distortion.

The resultant output of both resistors 36 and 3! is amplified at 62 and impressed'upon intensity grid 18. It will be recalled that if attenuators 30 and 33 are so adjusted that the voltages across resistors 36 and 3'! are equal, then the resultant output, as shown by curve 40 in Fig. -2, is zero in the center frequency region and negative on either side of this region. As a re-- sult the signal to which the receiver is accurately tuned will not change the bias on grid I8 and the signal will be visible. However, shoulda signal outside the center frequency region, but within the pass-bandof tuner 22 and broad band filter 34, be present, then the resultant negative potential will further increase the negative bias on grid 18 to an extent sulficient to blank or cut off the beam of the oscilloscope or at least considerably reduce the beam intensity. This mode of operation permits use of a-filter 3| having a fairly narrow band pass since a reasonable 'amountof distortion in the blanking signal circuits is not objectionable.

The circuit in Fig. 4 iscapable of sun another mode of operation. -By adjusting attenuators 30 and 33 (Fig. 2) so that the potential across -resistor 36 is greater than'that across resistor 31, theoutput will be positive within the region of the center frequency and negative outside this region, as shown by curve 4| in Fig. 3.- As a result, the negative potential on grid l8 will be reduced for desired signal frequencies, thus brightening the image, and increased for other frequencies, thus dimming or entirely blanking the image.

Fig. 5 shows in greater detail one typebf-bircuit operating in accordance with theprinciples set forth in connection with Figs. 2 and 4. The output of the "I. F. amplifier of the receiver is impressed upon the input grid of I. F. amplifier tube 16. The output of this amplifier is impressedpthrough a coupling condenser ll upon a broadly tuned circuit :12 tuned to the center frequency of the I. F. amplifier. The output of tuned circuit 12 is impressed through a variable coupling condenser 13 upon a rectifier circuit including the diode section 14 connected in parallel'with a high impedance load circuit including resistors 15 and 76in series. The cathode of this diode section is grounded.

Circuit 12 is also loosely coupled to a tuned circuit 11 through a link circuit including coils l8 and 19 coupled to circuits l2 and 11 respectively. The link circuit coils may be tapped or they maybe physically movable with respect to their companion coils to provide for variable coupling.

Circuit T1 is tuned to the same center frequency as circuit 12 but it has. a considerably higher Q, and hence aconsiderablynarrower passband than circuit 12. The output of circuit 11 is impressed, through a coupling condenser '80, upon a rectifier circuit, including diode section 8| in parallel with series connected load resistors 82 and 83. Condensers 84 and 85, "connected across load resistors 16 and 83, are R. F. by-pass condensers, having a low impedance for the R. F. components of the signal but a high impedance for the detected pulse components. Hence the pulse components appear across resistors l6 and 83.

In general, both rectifier circuits are iden tical, i. e. resistors l5 and 82 are equal, resistors &2. 41. l

I6 and 83 .are equal, andv condensers 84 and-85 are equal. The output of both load circuits can be derived separately or combined for purposes hereinafter described by means of two four position switches having their movable 'arms mecham'cally tied together as indicated by the dotted lines. 7

With the position of the switches. as shown, the potential across load resistor 16, which is developed by the R. F. voltage of broad band circuit 12, is impressed through leadlfi, switch arm 86, resistor 88 and condenser 89 upon a resistor 90. By making the time constant of the network including resistors 88 and 90 and condenser 89 short with respect to the pulse duration, the pulse potentials across resistor 90 will faithfully follow the envelope of the voltage in circuit 12. The output of. resistor 90 is amplified at 9| and impressed through couplingcondenser 92 upon the vertically deflecting-plates of oscil loscope D. .Thus, substantially, all signals which pass through broad band circuit 12 develop beam deflecting potentials. .-.It; is possible also to. use as beam deflectingpotential's the voltage across resistor 83 which is -de veloped by the output of narrow band circuit 11. :7 The potential, across resistor IS-is also combined in phase opposition to the voltage across resistor 83, in a circuit extending from ground. resistor 16, leads 1 6 and 91, switch arm 81, resistor 83 and lead 83. a I

Thepotential between lead 83' and ground is illustrated by the curves in Figure 6. These curves indicate the variation inpotential with frequency for different ratios of potential drops across resistors 16 and ,93. It will be seen that these curves are generally similar to those in Fig. 3. H

Curve in!) shows the volta e variation when the voltages across resistors l6 and 83 are equal at the center frequency F. Such equality can be obtained by adjusting thecoupling condenser-13. Curve I00 shows that at-center frequenc F the output voltage is .zero and ,at other frequencies within the pass-band of the=broad band ,circuit.

12, the voltage is negative. 1f condenser 13 is adjusted so that the voltage across resistor iiiiis greater at the center frequency F than that across resistor I6 then the voltage will vary as shown in curves H]! or 102, depending on the adjust F variable negative potential from potentiometer 99 applied to said grid through a high resistance The bias on .grid I8 is insufficient to block the beam. v If the circuits are so adjusted that the response is as shown by curve I00, then the blanking voltage impressed on grid I8 will bea .minimum for signals of frequency F or in the vicinity thereof. Hence the deflecting voltage developed at resistor 16 will deflect the beam to indicate the signal. If an interfering'signal in the region outside the vicinity of frequency F is present, then the negative voltage impressed on audit will be increased. to an. extent sufficient loscope.

to suppress the beam orgreatly reduce the bright-. ness thereof. I l

If the circuits are adjusted'so that the response is as shown bycurve I IDI or I02 then the signal applied to grid vl8 .will be positive for signals be; tween Fiand F2:so.that the .beam brightness will be increased. For signals outside this region the signal applied to said grid will be more negative so that the beam will be cut off or greatly reduced in' brightness. i V 4 The time .constantfof the network including condenser 94 and resistors 93,and 95 should pre'f-I erably be. slightly longer than thatinYthe deflec tion channel so that'the beam intensity control voltage applied to grid I3 should ear .sufiicient magnitude and duration to be. effective fort-at leastthe entire duratio'njof' the'b'eam deflecting voltage. 1 e I Switches 86 and .81 can operate in four'different positions, the one shown being the operate ing position. [The other three positions. areused for aligning thecircuits of the frequency disf modulated .variable frequency signalgene toris connected to theinput circuit offamplifier- 'lfl and the output of amplifier 9I-is applied tojthefvere tical plates of atest oscilloscopeLI The'frequency of the signal generatorshould be adjusted to the exact center frequency towhich the LF. ampli fier of the receiver is tuned, which represents frequency F in Fig. 6 f. l V1.1 f;

The first step is to adjust the'switches to. heir extreme counterclockwise position. This con nects the output acrcssresistor .16 to pul'seam'pli fier 9 I- through lead 16' and switch 86. Trimmer condenser 12' is now adjusted until ainaximum signal is obtained on, the, test oscilloscope. The second step1is to move th'e switches c1061;- wise totheir second position. This -connects'the output of resistor 83 to the test oscilloscope. Trimmer condensers l2 and 11' are now adjusted for maximum signal indic'ationbn thetest os The-thir d;. stepv is to move the x v i wise to their third position. 'Th seonne ts t combined output of resistors lfi a nd 83; I test oscilloscope for comparisonwith the prev ously indicated individual outputs. The fourth step is to ineve the. switc extreme clockwise position which is the n H operating position." The ,test oscilloscope is net disconnected from the output ofl arnplifierfl I nd connected to the output of amplifier 96.. 1

setting applies the outputs ofiresistors'lfia 8.3 in phase opposition to the vertical .platesiof. he test oscilloscope. Coupling condenser 'l 3 isnow adjusted untila small output appears abovethe base line of the test oscilloscope. Since adjustment of condenser 13 might detune circuit T2, the second stepshould nowbe repeated. The fife.- quency-of the test signal generator should now be slowly varied from a' point substantially below frequency =.F3 (Fig.6)- to a pointsubstantially above F4. As the frequency is varied the-amplitude of the signal on the test oscilloscope should vary'in' a manner indicated by curvelill-orfl'fli in Fig. 6. The extent of deflectio'nfbelow the base line should be much greater than the deflection above the 'ba se line. If this conditionis notobtained, thefourth step should be repeated with af'slightly different setting 16f 'coupling'cona denserl3. Q j I Terminals or 'pl'ugsand jacks (not shown) can be provided in thejcircuit for. convenienceincon- 9 nesting,- the signal: generator and test oscilloscope.

In one discriminator: circuit such as shown in Fig. 5., the following circuit constants were found suitable: Condenser 13 is variable through a range of 3 to 20 mid. Condenser 80 is 5 mmf. Condensers I6 and 83 are each 10 mmf. Condensers 84' and 85 are 10 mmf. Resistors I5 and 82 are each 10,000 ohms. Resistors I6 and 83 are each 15,000 ohms. The constants of circuits I2 and TI are such that they resonate at the intermediate frequency. It should be distinctly understood, however; that values other than those given may also be used, since they are not critical and can be changed to meet any given design factors.

Reference is now made to Figure '7 which shows astill further extension of the use of the frequency discriminating networks above discussed. This figure incorporates, in addition to the oscilloscope D, a plan position indicating oscilloscope P for giving a continuous indication of the direction and distance of all reflecting objects present. The remainder of the circuit is substantially the same as Fig. 6, the main difference being the use of variable potentiometers I16 and I83 as. diode load resistors. This permits, if desired use of a fixed coupling condenser 13, since adjustment of the combined voltages can be accomplished. solely by means of said potentiometers. For clarity, test switches 86 and 8'! havealso been omitted.

Use of the variable potentiometers permit more flexible control of" the combined voltages used for beam intensity control. The entire output across resistor H6 is amplified at S"! and applied as a beam deflecting voltage to oscilloscope D. A portion of the voltage across resistor I16 is tapped off by slider IT! and combined in opposition with the portion of the output of resistor I83 tapped off by slider I84. This combined voltage is amplified at 96 and applied to the intensity grid 18 of oscilloscope D. The operation of this portion of the circuit is exactly the same as that of Fig. 5.

The voltage across resistor I83 is combined in opposition with the voltage tapped oif by slider I". This combined voltage is amplified at I85 and applied to intensity control grid I85 of oscilloscope P. This grid is normally biased at or near cutoff by a negative voltage applied through resistor I81, said bias being sufficient to block the beam or permit a very small beam current so that a dim trace is visible upon the screen I88 of the oscilloscope. Since the signal output of amplifier I85 has a characteristic such as shown by curves WI or I02 in Fig. 6, a signal of the desired frequency will decrease the negative bias so that a bright spot I89. or I90 will appear on the screen. Undesired signal frequencies will render grid I86 more negative and still further reduce the intensity of, or entirely out off, the beam. Signal indication in this case is accomplished by intensity grid modulation alone rather than by beam deflection or a combination of the twoas is the case with oscilloscope D.

The beam is continuously radially deflected from the center of the screen I88 toward the periphery thereof by deflecting magnets IQI and I92 which are energized by the output of a sawtooth current generator !93 which is in turn controlled by synchronizing oscillator S (Figs. 1 and '7) This oscillator periodically keys a pulse generator I54, similar to H3 in Fig. 1, said generator in turn keying saw-tooth current generator I93.

Because of this radial deflection, the distance of a spot, such'as I89 or I90, fromthe center of the screen is a measure of the distance of the reflecting object. The screen of oscilloscope P is therefore graduated with distance indicating circles I94.

Means is also provided for rotation of the plane of beam deflection in synchronism with rotation of directional antenna 0. This is done by means of a motor which continuously rotates the yoke of beam deflecting magnets I9! and I92 around the axis of the tube, and synchronously therewith rotates antenna C to change the direction of radiation thereof. This synchronous rotationis indicated by the dot-dash line which represents either direct mechanical connection or equivalent servo-motor means.

I In this manner the plane, of beam deflection is at all times synchronized with the direction in which the antenna is effective so that the angle of the radial line 200 through the spot I89 with respect to a fixed directionis an indication of the azimuth of the reflecting object.

To provide for continuous indication of all reflecting objects or targets, the minimum speed of rotation of the antenna and magnet yoke, i. e. the speed of scanning, must be consistent with the persistence of the screen of oscilloscope P. High persistence screens permit speeds as low as 5 R. P. M.

Rotation of the beam deflection plane can be accomplished by means other than the use of a rotating magnet yoke. For instance, rotating electric or magnetic fields can also be obtained by static methods. For this purpose a Selsyn generator (not shown) driven in synchronism with the antenna, can be used to develop two currents, phase displaced ninety degrees with respect to each other; Both currents are then mixed with the output of sweep current generator I93. One of the mixed currents is. then applied to one'pair of stationary deflecting magnets and the other mixed current is applied to a second pair of stationary deflecting magnets, one pair deflecting the beam in plane perpendicular to the plane of deflection of the other pair. This creates a rotating magnetic field which rotates the beam. For a more detailed showing of one such beam rotating circuit reference is made to Patent No. 2,313,966, issued to W. J. Pooh.

Oscilloscope P indicates positions in terms of polar coordinates. The system shown in Fig. '7 can also be used with oscilloscopes giving positions in terms of rectangular coordinates. In fact, it can be used with any known type of oscilloscope in which signals control the beam intensity.

The use of both Oscilloscopes P and D is of advantage since th amplitude of the incoming signals can be more readily observed on oscilloscope D. However, the latter can be eliminated where circumstances do not require it.

The circuits in Figs. 4, 5, and '7 reduce or entirely eliminate the oscilloscope indication upon the occurrence of an interfering signal within the pass band of the receiver but outside the pass band of the narrow band filter. Under certain conditions, this will affect the reproduction of the desired signal if interfering signals are present at the same time. However, since most signals of this type are pulse modulated, the desired pulse signals will still be reproduced, unless the interfering pulses are in exact synchronism with the desired pulses; an occurrence wh ch s ext emely unlikely. If this should occur,

'uring circuits.

11 however, it is a simple matter to change the phase or repetition rateof the desired pulses.

It should be understood that the circuits above described are not restricted for use with radio object location systems nor with the particular indicating means shown. They are equally applicable to other radio systems using intermittently modulated signals. They are also applicable to systems using other than radio waves, e. g. systems using sound waves in air or water for communication, object location, or depth sounding. In addition, the frequency discriminator circuits are equally applicable to any electrical system requiring a discrimination between frequency, e. g. frequency indicating and meas- Finally, the discriminator voltages can be applied as a bias on the tubes of the receiver channels instead of the oscilloscope. Or it may be applied to both portions of the system.

While there have been described what are at present considered preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications 1 may be made therein without departing from the invention, and it is therefore, aimed in the appended claims to cover all such changes and modifications as ,fall within the true spirit and scope of the invention.

I claim:

1. A superheterodyne receiver system for interrupted waves comprising an intermediate frequency channel having a relatively wide band pass, at least two tuned detector networks tuned to the center frequency of said channel and connected thereto, one of said networks having a band pass at least equal tothat of said channel, the other network having a band pass narrower than said channel, an oscilloscope having a beam intensity control'electrode, means to combine the outputs of both networks in opposition to produce a resultant output and means for impressing said resultant output upon said intensity control electrode.

2. A superheterodyne receiver systemfor interrupted waves comprising a tuned intermediate frequency channel having a relatively wide band pass, at least two tuned detector networks tuned to the center frequency of said channel and coupled thereto, one of said networks having a' band pass narrower than the other, an oscilloscope having a beam intensity control electrode and at least one deflecting electrode, means to impress the output of one of said networks upon said deflecting electrode, means to combine the output of both networks in opposition to produce a resultant output, and means to impress said resultant output upon said intensity control electrode.

3. A superheterodyne receiver system for pulse modulated continuous waves comprising a tuned intermediate frequency channel having a relatively wide band pass, at least two tuned detector networks tuned to the center frequency of said channel and coupled thereto, one of said networks having a band pass at leastas wide as said channel and the Othernetwork having a band pass narrower than said channel, an oscilloscope having a beam intensity control electrode and at least one deflecting electrode, means to impress the output of the broader band network upon said deflecting electrode, means toequalize the outputs of both networks at the center frequency, means to combine said outputsin opposition, and means to amplify said combined output and impress it upon said intensity control electrode in such direction as to decrease the beam intensity.

4. A frequency discriminating network comprising two circuits tuned to the same frequency, one of said circuits being more sharply tuned than the other, means to separately rectify the output of each circuit, means for combining the rectified outputs in opposition, an indicator, and switching means for individually selecting'the rectified output of either circuit or said combined output and impressing the selected output on the indicator.

5. A superheterodyne receiver system for pulse modulated continuous waves comprising a tuned intermediate frequency channel having a relatively wide band pass, at least two tuned detector networks tuned to the center frequency of said channel and, coupled thereto, one of said networks having a band pass at least as wide as said channel and the other network having a,

band pass narrowerthan said channel, an oscilloscope having a-beam intensity control electrode and at least one deflecting electrode, means to impress the output of the broader band network REFERENCES CITED The following references are of record in'tlie file of this patent:

UNITED STATES PATENTS Number Name Date 2,053,762 Braden Sept. 8, 1936 2,113,212 Landon Apr; 5, 1938 2,127,816 Holst et a1. Aug. 23, 1938 2,153,780 Van Loon Apr. 11, 1939 2,186,867 Jeffcock Jan. 9, 1940 2,206,010 Braselton July 2, 1940 2,210,738 Tubbs Aug-6, 1940 2,243,140 Weagant May 27, 1941 2,262,218 Andrews Nov. 11, 1941 2,355,363 Christaldi Aug. 8, 1944 2,367,907 Wallace Jan. 23, 1945 2,416,346 Potter Feb. 25, 1947 2,426,580 OBrien Aug. 26, 1947 FOREIGN PATENTS Number Country Date 337,050 Great Britain Oct. 27, 1930' 451,227 Great Britain July 31, 1936 OTHER REFERENCES Rider, Cathode-Ray Tube at Work, page 172, John F. Rider Publication, 1935. 

